If you are looking for the external cables you can find them on Apples web site here Apple - Cables Apple sells a few third party cables intermixed with theirs, mostly on the older less asked for cables.
There are alternative local sources which might offer what you need (and cheaper). Also keep in mind the newer Thunderbolt-3 ports on your system also support USB-C cables. Many people find getting a dongle or hub device offers the needed conversion to what your display needs.
If you are looking for an internal cable you'll need to post a picture so we can identify it properly Adding images to an existing question
OK so you need the internal displays data cable 21.5” iMac Embedded Display Port Cable, apple P/N 923-01669